Structures useful in electron beam lithography

ABSTRACT

A method for forming a mask assembly for use in lithography, typically electron-beam lithography, first forms in a substrate one half of a plurality of opening therethrough and then fills the openings with a removable fill material. Thereafter are formed the other half of the openings which are then filled with the removable fill material. After all the openings have been formed and filled, a support membrane is formed over the substrate and covers the filled windows. A mask layer is then formed over the membrane and patterned. The fill is then removed from all of the windows.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.09/698,706, filed Oct. 27, 2000 which issued as U.S. Pat. No. 6,746,805on Jun. 8, 2004. This application is related to U.S. Ser. No.09/968,994, entitled “Methods For Fabricating Stencil Masks”, which hasa common assignee and one common inventor with this application andwhich was filed concurrently with U.S. Ser. No. 09/698,706.

FIELD OF THE INVENTION

This invention relates to lithography, and more particularly, to masksused in lithography and mask support structures and their preparation.

BACKGROUND OF THE INVENTION

As the size of electronic components used in integrated circuits hasshrunk and their density in circuits increased, there has been growinginterest in electron beam lithography. In electron beam lithography, anelectron beam is used to write patterns on an electron beam resist layerformed over a top surface of the semiconductor wafer that is theworkpiece for the electron beam lithography. A particular form ofelectron beam lithography of special interest for the invention isdescribed as Scattering With Angular Limiting Projection Electron BeamLithography (“Scalpel”).

Scalpel is an electron beam lithography technique that typically hasemployed masks in which the mask used for control in the irradiation ofthe workpiece has been formed as a thin patterned coating supported on athin membrane. Scalpel has been described in various publications.

The requirements for such a membrane-mask combination are quitedemanding. Typically, it needs to be planar, relatively easy to make,and comparatively rugged. A particular problem has been the forming ofthe membrane that is to support the coating in which the mask pattern isformed. The membrane needs to be of relatively large area, typically atleast several inches on a side, to be practical for use in large scalemanufacture. It needs also to be very thin, typically no thicker thanabout 1000 Angstroms in thickness, to permit electrons that pass throughthe mask to penetrate it easily without excessive scattering orexperiencing excessive loss in electron beam energy. Moreover, it alsoneeds to remain planar with inappreciable sag in use so that itcontinues to support the mask uniformly over its entire area.

To insure that the membrane remains sag-free, it is generally thepractice to suspend the membrane by a substrate that provides a suitableunderlying support grillage, typically consisting of major struts andminor struts (ribs).

Hitherto, for forming the grillage for supporting the membrane thatsupports the mask coating, a layer of silicon nitrite was deposited overthe top surface of a silicon wafer and then the silicon wafer was etchedto leave a portion thereof to provide the supporting grillage for thenitride coating that was to serve as the membrane. However, this hasproven difficult.

One problem with this approach has been the difficulty of maintaining,during the shaping of the silicon wafer, the mechanical stability of thegrillage left to support nitride the membrane. The physical dimensionsof commercially available silicon wafers limits formation of thin strutsneeded.

The present invention involves a better way to prepare a mask useful inelectron beam lithography.

SUMMARY OF THE INVENTION

In a first aspect the invention is a method for forming a mask assemblyfor use in lithography. The method comprises the steps of: forming asupport structure that comprises a substrate that includes a pluralityof windows filled with a temporary fill; forming over thefilled-windowed substrate a mask; and removing the temporary fill.

In a second aspect the present invention is a method for forming a maskassembly for use in lithography. The method comprises the steps of:forming a support structure that comprises a substrate that includes aplurality of windows filled with a temporary fill; forming over thefilled-windowed substrate a membrane layer for supporting the masklayer; forming a mask layer over the membrane layer; and removing thetemporary fill.

In a third aspect the invention is a method of forming a mask assemblythat comprises the steps of: forming in a substrate a support structure,which includes major and minor struts that define an array of windows ina two-dimensional array of rows and columns, by successive rounds ofcutting in the substrate a fraction of the total window area to beformed; filling such fraction of windows with temporary fill before thesucceeding round of cutting and filling until all the window areas arecut and filled; forming a membrane layer over a top surface of thesupport structure; forming a mask layer over the membrane layer; andremoving the fill from the windows.

In a fourth aspect the present invention is a method of forming a maskassembly for use in electron beam lithography that comprises the stepsof: forming in a substrate a first set of spaced-apart windows; fillingthe windows with a temporary fill; forming in the substrate a second setof windows in the spaces between the first set of windows for formingwith the first set a two-dimensional array of windows arranged in rowand columns; filling the second set of windows with a temporary fill;depositing over the filled-windowed substrate a layer suitable forsupporting a mask; depositing over the last-mentioned layer a layersuitable for providing a mask; patterning the last-mentioned layer toform a mask, and removing the temporary fill from the windows, wherebythe mask layer is free of underlying substrate.

In a fifth aspect the present invention is a method for forming a maskassembly comprising the steps of: forming by use of a mold a supportstructure that defines an array of windows arranged in rows and columns;filling the openings with a temporary fill; forming over the supportstructure a membrane layer; forming over the membrane layer a patternedmask; and removing the temporary fill.

In a sixth aspect the invention is a method of forming a mask supportstructure that comprises the steps of: forming in a substrate a firstset of spaced apart windows; filling the first set of windows with atemporary fill; forming in the substrate a second set of windows locatedin portions of the substrate between the first set of filled windows;and filling the second set of windows with a temporary fill.

In a seventh aspect the invention is a method of forming a mask supportstructure that comprises the steps of: placing in a mold which is shapedto facilitate the formation of a support structure a plurality ofparallel minor struts; and forming in the mold a mask support structurethat comprises a frame and plurality of major struts that are orthogonaland attached to the minor struts with the major and minor strutsdefining a plurality of windows arranged in a two dimensional array ofrows and columns.

From a product aspect, the invention is the product of the variousmethods described.

The invention will be better understood from the following more detaileddescription in conjunction with the accompanying drawing and claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1. shows a top view of a mask support structure in accordance withthe invention suitable for use in electron beam lithography;

FIG. 2 shows a cross-sectional view taken along a dashed line 2—2 ofFIG. 1 with a membrane layer over the mask support structure;

FIGS. 3, 4, 5, and 6 show steps in the fabrication of the mask supportstructure of FIG. 1;

FIG. 7 shows a cross sectional view taken along a dashed line 7—7 ofFIG. 6;

FIG. 8 shows the section of FIG. 7 after there has been deposited amembrane over a top surface thereof;

FIG. 9 shows the section of FIG. 8 after there has been deposited overthe membrane a layer that will be patterned to serve as the mask forelectron-beam lithography;

FIG. 10 shows the section of FIG. 8 after the fill has been removed fromthe windows;

FIGS. 11-14 show cross sectional views that illustrate an alternativemethod for providing the membrane and mask layers over a mask supportstructure of the kind shown in FIG. 7; and

FIGS. 15, 16, and 17 shown a top view, a first cross-sectional viewthrough a dashed line 51—51 of FIG. 15, and a second cross-sectionalview through a dashed line 52—52 of FIG. 15 of a mold useful in theproduction of a mask support structure in accordance with anotherembodiment of the invention;

FIG. 18 shows the resulting mask support structure after it is removedfrom the mold;

FIG. 19 shows a three-dimensional view of a mask support structure in anearly stage of fabrication in accordance with another embodiment of thepresent invention;

FIG. 20 shows the mask support structure of FIG. 19 at a succeedingstage of fabrication; and

FIG. 21 shows a cross-sectional view taken along a dashed line 2—2 ofFIG. 1 with a stencil mask over the mask support structure;

The drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 and 2, there is shown a mask support structure10 for use in electron beam-lithography in accordance with the inventionwith FIG. 1 showing a top view and FIG. 2 showing a cross-sectioned viewthrough a dashed line 2—2 of FIG. 1. Mask support structure 10 comprisesa substrate 12 having an outer frame portion 12 a, advantageously ofsquare shape, major struts 12 b, and minor struts 12 c. The outer frameportion 12 a can be any shape, for example, circular. The major strutsare vertical regions and the minor struts are horizontal regions. Majorstruts 12 b and minor struts 12 c are orthogonal to each other anddefine a plurality of windows (openings) 18, arranged in an array ofrows and columns. The major struts 12 b are at least several timesthicker than the minor struts 12 c.

As seen in the cross-sectioned view of FIG. 2, over a top surface of themask support structure is a thin membrane layer 20 that is to support alayer (not shown) that will have been patterned to serve as a masklayer. Mask support structure 10 with the membrane layer 20 on topthereof and a patterned mask layer 28 (shown in FIGS. 9-14) over themembrane layer 20 may be denoted as a mask assembly. The mask layertypically is of a material and thickness to scatter electrons tosufficient angles that it intercepts where unpatterned to be useful inelectron beam lithography, but allows electrons to pass through theopenings in the pattern little impeded. The membrane 20 needs be of amaterial and thickness adequate to support the mask layer withoutsagging but to little affect the passage of electrons therethrough.

In accordance with the invention, the mask support structure assembly 10is prepared in a novel fashion. To this end, as is shown in FIG. 3 thereis provided a substrate 12 of a suitable material and advantageously aceramic, such as aluminum oxide or silicon carbide, of sufficientthickness to be rigid and self supporting, in which are shown by solidlines a first set of windows 18 a that are cut in a first round ofcutting. In this first round, there are cut one half of the total numberof windows to be cut. Advantageously all the windows to be cut are toform a two-dimensional array in rows and columns, as shown in FIG. 1,and in this first round, there are cut alternate windows 18 a in eachrow and column as is denoted by the solid lines. Also shown in FIG. 3 bydashed lines are the regions where windows 18 b of the second set thatare to be cut in the second round of cuttings. These will form with thefirst set 18 a the two-dimensional array of windows. In one example thewindows 18 a and 18 b are each about 12 millimeters by 1.1 millimeters,and are formed by laser cutting.

After the first set of half of the window pattern has been cut out toform windows 18 a, these windows 18 a are temporarily filled with asuitable material that can later be conveniently introduced and laterconveniently removed, advantageously an epoxy, silicon oxide, a polymer,or a metal such as aluminum. The result is seen in FIG. 4 where thefirst set of filled window regions 18 a is shown stippled.

Next there are cut out the remaining windows 18 b of the second set, asis shown in FIG. 5, typically in the same manner as before.

Then the second set of windows 18 b are filled typically in the mannerused previously to fill the first set. The result is shown in a top viewin FIG. 6 and in a cross-sectional view through a dashed line 7—7 ofFIG. 6 in FIG. 7. All of the window regions 18 a and 18 b are shownfilled, as is indicated by the stippling in both windows 18 a and 18 b.

Next, the opposite major surfaces of windowed substrate 12 are groundand polished to provide a mask support structure with smooth, planar,and parallel opposite major surfaces. This can be done, for example, bychemical mechanical polishing (CMP).

As mentioned earlier, there are various ways consistent with theinvention in which the membrane layer 20 that will support a mask layer28 can be provided on the mask support structure 10.

In one method there is deposited, typically by chemical vapordeposition, over a major surface of the mask support structure 10 alayer of the material that will form a membrane layer 20 for supportingthe mask layer 28 (shown in FIGS. 9-14), as is shown in thecross-section view of FIG. 8. Suitable membrane materials includesilicon nitride, silicon, silicon carbide, diamond, and aluminum oxide.The membrane should be of a thickness sufficient to adequately supportthe mask layer but to little impede the passage therethrough ofelectrons that pass through the patterned mask layer. A thickness of1000 Angstroms or less should be suitable.

Next, as is shown in the cross-sectional view of FIG. 9, there isdeposited over a top surface of the membrane layer 20 a layer that willbe patterned to serve as a mask layer 28. The mask layer 28 typically isof a material and thickness to scatter electrons to sufficient anglesthat it intercepts where unpatterned to be useful in electron beamlithography, but allows electrons to pass through the openings in thepattern little impeded. Examples include tungsten, and tantalum siliconnitride, about 300 Angstroms thick.

Next, the mask layer 28 is patterned appropriately in any suitablemanner. Typically this is done by first coating it with a standardelectron beam resist, and then patterning the resist coating in thedesired mask pattern with an electron beam. Then this mask pattern istransferred in the usual manner from the resist coating into the masklayer 28, after which the resist coating is removed to leave a patternedmask layer 28 as a coating on the membrane.

Now, there is removed the fill material from the windows 18 a and 18 bto open them. The result is shown in FIG. 10 in which the fill has beenremoved from the windows 18 a and 18 b formed between the major andminor struts 12 b and 12 c and the mask layer 28 has been patterned toserve as a mask. Advantageously the fill material was suitably chosen topermit its easy removal, as by wet chemical etching without damage tothe mask. In some instances, it may be possible to remove the fill inthe same process used to remove the unneeded portions of the mask layer.In specific cases, the fill may have been removed just before the masklayer was patterned, unless the additional support the fill provides wasstill needed during the patterning.

This results in a mask support assembly with a mask layer 28 thereon.

It can be appreciated that a fraction different from one half, forexample a third, appropriately chosen to maintain adequate rigidity, ofthe total number of windows to be cut, can be cut (formed) and filled inseparate rounds during the formation of such windows Additionally, itcan be appreciated that rather than a fraction of the total number ofwindows to be formed can be cut and filled in each round, a fraction ofthe area of each window to be formed can be instead cut and filled insuccessive rounds, to maintain the desired strength of the substrateduring the window-cutting steps.

As was also mentioned above, various other methods consistent with theinvention can be used to provide a membrane layer 20 for supporting themask layer 28 on the mask support structure 10 shown in FIG. 7. Forexample, in another method there is prepared a suitable second substrate30, for example, of silicon, typically of the same size and shape as thesubstrate 12. As shown in FIG. 11, one major surface of the substrate 30is covered in turn with an underlying layer of a thickness and materialsuited for use as the mask layer 28 and an overlying layer of a size andmaterial suitable for use as the membrane layer 20.

Then, as is shown in FIG. 12, the second substrate 30 is bonded by wayof its membrane layer 20 to a major surface of the filled-window masksupport structure 10 of FIG. 7 by any suitable means, as by plasmaactivation bonding (not shown) or the use of an adhesive layer (notshown).

Then there is removed the second substrate portion 30, as by etching, toleave the structure shown in FIG. 13, in which mask layer 28 is exposed,supported on the first substrate 12 by way of the membrane layer 20.

There can now be patterned the exposed mask layer 28, in the mannerpreviously described.

There can now be removed the fill in the supporting grillage to leavethe structure shown in FIG. 14, which is the structure desired for maskassembly.

To facilitate the removal of the second substrate 30, it can beion-implanted before bonding to the mask support structure 10, to form adefect-rich buried layer in its interior. The ions could, for example,be hydrogen or oxygen. This makes it is easy to cleave the substrate 30along such defect-rich layer. The cleaving of the substrate 30 canappreciably thin the substrate 30 and thus facilitates its subsequentcomplete removal to expose the layer that is to form the mask layer 28.The dashed line 36 across substrate 30 in FIG. 12 illustrates thelocation of such an optional ion-implanted layer within substrate 30.

Referring now to FIGS. 15, 16, and 17, there are shown a top view (FIG.15), a first cross-sectional view (FIG. 16) through a dashed line 16—16of FIG. 15, and a second cross-sectional view (FIG. 17) through a dashedline 17—17 of FIG. 15 of a mold 50 useful for forming a supportstructure in accordance with another embodiment of the invention. Themold includes wide openings for defining the major struts and narroweropenings (grooves) for defining the minor struts. A plurality ofparallel minor struts (elongated strips) 54 have been placed in thegrooves of the mold 50. When mold 50 is filled with a molding material,the molding material attaches itself to minor struts 54. Grooves in themold 50 have a depth (height) of “d” (See FIGS. 16 and 17). Minor struts54, which typically has a depth less than “d”, can optionally be of thesame depth (height) d as the grooves in the mold 50. The material forthe minor struts 54 and the mold-filling material can be the same ordifferent materials. After the fill material hardens a mask supportstructure 100 (see FIG. 18) is formed in the mold 50.

FIG. 18 shows the mask support structure 100 after there has beenremoved from the mold 50. Mask support structure 100 has a frame 56comprising major struts 56 a and minor struts 54 that define rows andcolumns of an array of openings (windows) 58. Mask support structure 100is similar to mask support structure 10 of FIG. 1. It is thenadvantageously filled with a temporary fill (not shown) and thenopposite major surfaces are planarized. Mask support structure 100 isthen used as a support for a membrane layer and a mask layer inessentially the same manner as explained previously for supportstructure 10 of FIG. 1. Still another method of forming a suitable masksupport structure for use in the invention is described below withreference to FIGS. 19 and 20.

Referring now to FIG. 19, there is shown a three-dimensional view of aframe 60 which has a plurality of spaced apart large rectangular opening62 formed therethrough. Only three openings 62 are shown. Frame 60 isuseful for forming a support structure in accordance with an otherembodiment of the invention. A plurality of parallel grooves 64 areformed that extend across the frame including the regions betweenopenings 62. Grooves 64 can optionally be of the same depth (height) asthe openings 62.

Referring now to FIG. 20, a separate minor strut 66 is inserted intoeach of the grooves 64 to divide the large openings and to form a masksupport structure 1000 in accordance with an other embodiment of theinvention. Mask support structure 1000 of FIG. 20 is similar to supportstructure 10 of FIG. 1 and to mask support structure 100 of FIG. 18 andcan be used in the manner previously described for such mask supportstructures. It is then advantageously filled with a temporary fill (notshown) and then opposite major surfaces 60 a thereof are planarized.Mask support structure 1000 is then used as a support for a membranelayer and a mask layer in essentially the same manner as explainedpreviously for mask support structure 10 of FIG. 1.

Referring now to FIG. 21, there is shown a cross-sectional view of themask support structure 10 of FIG. 1 with a stencil mask 21 over a topsurface of support structure 10. Stencil mask 21 serves same the purposeas membrane layer 20 (shown in FIG. 2) and the mask layer 28 shown inFIGS. 9-14 and can be substituted for membrane layer 20 and mask layer28. Stencil mask 21 has formed therethrough a plurality of openings 21 awhich define a pattern. Stencil mask 21 can be used with the masksupport structure 100 of FIG. 18 and with the mask support structure1000 of FIG. 19.

It can be appreciated that the principles of the invention areessentially independent of the specific materials employed and of thespecific dimensions described. For example, subsequent developments inmaterials and in cutting and filling techniques will likely affect whatis preferred in the future. Still further, it should be apparent that inthe embodiment involving successive rounds of cutting and filling thatit may not be necessary to interleave all of the window opening ofdifferent rounds since it should be feasible in some instances toinclude in the same cutting round pairs or more of adjacent windows ofthe window pattern desired.

1. A mask assembly for use in electron beam lithography, wherein saidmask assembly is formed by a method comprising the steps of: forming asupport structure that comprises a substrate that includes an initialplurality of windows; filling the initial plurality of windows with atemporary fill material; forming an additional plurality of windows inportions of said substrate which do not contain the temporary fillmaterial; filling the additional plurality of windows in the substratewith a temporary fill material; forming over the filled-windowedsubstrate a mask structure which includes a supporting membrane whichpermits transmission of electrons therethrough and a patterned masklayer overlying the supporting membrane; and essentially completelyremoving the temporary fill material, whereby a mask assembly isproduced which comprises a windowed substrate containing a plurality ofopen windows, with a supporting membrane which transmits electronsdirectly overlying, parallel to, and supported by a major surface ofsaid windowed substrate, and a patterned mask layer directly overlyingthe support membrane, wherein said mask assembly width and lengthdimensions are each several inches on a side.
 2. A structure useful inelectron beam lithography for controlling the irradiation of asemiconductor workpiece, said structure including a masking assemblywhich is supported by a grid structure, said grid structure comprising aplurality of windows which permit the transmission of electron beamradiation passing through said masking assembly, where the maskingassembly includes a membrane layer which permits electron transfertherethrough and a mask layer which overlies the membrane layer and issupported by the membrane layer, wherein a plurality of major struts andminor struts arranged in rows and columns define an array of windows,wherein said major struts are several times thicker than said minorstruts, and wherein said minor struts exhibit a width which does notinterfere with transmission of electron beam radiation which has passedthrough said mask.
 3. A structure in accordance with claim 2, whereinsaid membrane permits electrons to pass through said membraneessentially without loss in electron beam energy.
 4. A structure inaccordance with claim 3, wherein the thickness of said membrane is lessthan about 1000 Angstroms.
 5. A structure in accordance with claim 2,wherein said mask layer is a stencil mask.
 6. A structure in accordancewith claim 2, or claim 3, or claim 4, or claim 5, wherein said pluralityof struts is constructed from a ceramic material.
 7. A structure inaccordance with claim 6, wherein said ceramic material is selected fromthe group consisting of aluminum oxide and silicon carbide.
 8. Atemporary structure useful in fabrication of an electron beamlithography mask, said temporary structure comprising a patterned masklayer overlying a membrane support layer overlying a grid structurewhich includes a plurality of windows which are filled by a supportmaterial, wherein said plurality of windows comprises a plurality ofmajor struts and minor struts, and wherein said major struts are severaltimes thicker than said minor struts.
 9. A temporary structure useful infabrication of an electron beam lithography mask, said temporarystructure comprising a patterned mask layer overlying a membrane supportlayer overlying a grid structure comprising a plurality of windows whichare filled by a support material, wherein said membrane layer permitselectrons which pass through the patterned layer to pass through saidmembrane essentially without loss in electron beam energy.
 10. Atemporary structure in accordance with claim 9, wherein the thickness ofsaid membrane is less than about 1000 Angstroms.
 11. A temporarystructure in accordance with claim 8 or claim 9, wherein said mask layeris a stencil mask.
 12. A temporary structure in accordance with claim 8or claim 9, wherein a plurality of struts is constructed from a ceramicmaterial.
 13. A temporary structure in accordance with claim 12, whereinsaid ceramic material is selected from the group consisting of aluminumoxide and silicon carbide.
 14. A temporary structure in accordance withclaim 12, wherein a temporary fill material is selected from the groupconsisting of an epoxy, a polymer, a metal, or silicon oxide, whereinthe selection of said temporary fill material depends on the selectionof material used to construct said plurality of struts, so that saidtemporary fill material can be conveniently removed essentially withoutharm to said plurality of struts.
 15. A temporary structure inaccordance with claim 13, wherein a temporary fill material is selectedfrom the group consisting of an epoxy, a polymer, a metal, or siliconoxide, wherein the selection of said temporary fill material depends onthe selection of material used to construct said plurality of struts, sothat said temporary fill material can be conveniently removedessentially without harm to said plurality of struts.